Silverlight 5 has released! Here's the link to download the Silverlight 5 tools.
Silverlight 5 offers many new features including:
For a detailed list of the new features see the Silverlight content on MSDN, specifically, the What's New for Silverlight 5 topic.
If you prefer your docs offline, here's the link to download the .chm containing Silverlight 5 docs.